As is well known, fluorescent lamps have a number of advantages over incandescent lamps, including lower power consumption per light output, lower heat output (and, thus, lower air-cooling load), and longer useful life-- and, therefore, lower replacement labor cost. Fluorescent lamps are now being provided with screw-in Edison bases for use as replacements for incandescent lamps. A fluorescent lamp system requires-- in addition to a lamp bulb itself-- a "ballast" to ensure proper starting and operating conditions.
Iron-core inductive ballasts, though customary for fluorescent lamps for many years, are being superseded by solid-state ballasts. Representative disclosures of solid-state ballasts in U.S. Pat. Nos. include Stoltz Pat. 4,251,752; Stevens Pat. 4,277,728; Knoll Pat. 4,109,307; and Perper Pat. 4,017,785; plus references cited therein.
Solid-state ballasts not only enable flicker-free, hum-free fluorescent lighting but also are more compact, lighter in weight, and adapted to controlling lamp operation in ways not possible with iron cores. Included among conditions desirably controlled via the ballast are voltage, current, frequency, and light output.
Even solid-state ballasts may have such faults as operating a lamp at hotter than optimal temperature, or providing harmonic-laden inputs of limited contribution to light output, and/or lacking the capability of being dimmable as incandescent lamps are-- over a broad range and whenever maximum light output is unnecessary or unwanted.
Dimming with solid-state ballasts is disclosed in such U.S. Pat. Nos. as Capewell et al. 4,207,497; Spira et al. Pats. 4,207,798 & 4,350,935; Zansky Pat. 4,370,600; Hyltin Pat. 4,388,563; Burke Pat. 4,686,427; and Summa 4,859,914 -- by such methods as varying the amplitude and/or the duty cycle of the input to or the output from the ballast. However, a need exists for a more versatile ballast, preferably with multiple dimming modes.